1. Field of the Invention
The present invention relates to non-aqueous electrolyte secondary batteries employing organic electrolyte, solid polymer electrolyte or the like, and more particularly to negative electrode materials thereof featuring a high capacity, high reliability for the safety aspect, a little decrease of discharge capacity resulting from cycles, and excellent high-rate charge and discharge characteristics.
2. Description of the Prior Art
A lithium battery employing lithium as negative active material, in general, generates high electromotive force and can be high energy density. The lithium batteries thus employ various positive active materials combined with the negative active material, and are commercialized to be battery systems, thereby reducing dimensions as well as weights of cordless and portable products.
A lot of effort made both for R&D and commercializing of the lithium batteries is not only for the primary battery, i.e., the battery should be replaced after discharging, but also for the secondary battery, i.e., the battery can be repeatedly used by recharging.
Lithium reacts with water excitedly, and produces hydrogen. Thus, the electrolyte of the lithium batteries employs non-aqueous electrolyte, e.g., 1) organic electrolyte made of dehydrated aprotic organic solvent dissolving lithium salt, and 2) solid polymer electrolyte. The lithium batteries are thus sometimes called non-aqueous electrolyte primary battery and non-aqueous electrolyte secondary battery.
As a positive active material of the non-aqueous electrolyte secondary battery, transition metal element oxides including vanadium pentoxide (V2O5), titanium disulfide (TiS2), molybdenum disulfide (MoS2) and chalcogenide were examined at first; however, recently, lithium contained transition metal element oxides of which transition metal element is partially replaced with another element have been examined. The oxides of lithium contained transition metal element such as LiCoO2, LiNiO2, LiMn2O4 which are double oxides of lithium and cobalt, lithium and nickel, and lithium and manganese. These double oxides repeat desorption and absorption of lithium ion by charge and discharge, whereby excellent cycle life characteristics can be obtained. The chalcogenide, one of the lithium contained transition metal element, is also examined besides these double oxides.
On the other hand, regarding the negative electrode materials, metal lithium which is active material could be the most desirable material from the standpoint of energy density provided it can be used as it is, because an electric potential would be least-noble. However, when this secondary battery is charged, active dendrite or mossy crystal having a large specific surface area deposits on the negative electrode surface, and the crystals thereof react with the solvent in the electrolyte, whereby being deactivated with ease, which lowers the capacity of the battery rapidly, and therefore, a larger quantity of metal lithium must be pre-filled in the negative electrode. Further, the deposited dendrite might penetrate the separator and cause an internal short circuit. These problems shorten the cycle life and affect the product safety.
In order to prevent the dendrite from occurring when the battery is charged, the following materials were tested as a negative electrode material: Li--Al alloy, alloy of Li and Wood's metal which is fusible alloy. When one of these metals which can be alloyed with lithium or such an alloy which contains at least one of these metals is employed as a negative electrode material, the battery indicates a relatively higher capacity at an initial charge and discharge cycle.
However, alloying with lithium and lithium elimination are repeated due to charge and discharge, and thus, a phase of the crystal is sometimes changed although a crystal structure of an original skeletal alloy is still maintained, or the crystal structure per se may be changed into a different one from the original skeletal alloy. In such a case, particles of the metal or the alloy, i.e., a host material of lithium which is an active material repeats swelling and shrinking. Along with the progress of charge and discharge cycles, a crystal particle of the metal or the alloy gets cracks, and the particle's fineness progresses finer and finer. Due to this phenomenon, an ohmic resistance between the negative electrode materials increases, and a resistance polarization grows during charge and discharge. As a result, these materials were proved not to satisfy a cycle life characteristic necessary for a practical use.
In recent years, a carbon material such as graphite is employed as a host material of the negative electrode, because such a carbon material is able to absorb and desorb lithium ion by charge and discharge. The carbon material as a negative electrode material, the lithium contained cobalt oxide as a positive electrode material and organic electrolyte are combined to make a system called lithium-ion secondary battery, which is now available in the market.
Further, in order to increase a negative electrode capacity, it is proposed in Japanese Patent Application Laid Open No. H07-315822 that a compound of a host material comprising graphitized carbon material and a material, e.g., silicon mixed product incorporated with the host material be employed as a negative electrode material. This laid open application teaches that the battery employing the proposed material has the higher capacity and the longer cycle life than those of the battery which employs silicon alone as a host material of he negative electrode. However, it seems that there is a poor chemical bonding force between silicon and carbon, whereby dimensional swelling of silicon due to absorbing lithium therein cannot be suppressed completely by the carbon around the silicon. As a result, a satisfactory cycle life is not realized.
Other materials are also proposed in the Japanese Patent Application Laid Open Nos. below in order to achieve a high capacity and a long cycle life:
H05-159780: Iron silicide, e.g., Fe2Si3, FeSi, FeSi2 PA1 H07-240201: transition element and nonferrous metal silicide, PA1 H09-63651: a host material comprising an intermetallic compound including at least one of 4B element, P or Sb and having one of a crystal structure of CaF2 type, ZnS type or AlLiSi type. PA1 (a) a core formed by solid phase A, and PA1 (b) solid phase B wrapping the core entirely or partially. PA1 (a-1) lithium, PA1 (a-2) at least one element which is able to alloy with lithium, PA1 (a-3) solid solution including an element which is able to alloy with lithium, or PA1 (a-4) intermetallic compound including an element which is able to alloy with lithium. PA1 (b-1) solid solution including (b-3) PA1 (b-2) intermetallic compound including (b-3) PA1 (b-3) at least one element which can alloy with lithium. PA1 zinc, cadmium, aluminum, gallium, indium, thallium, silicon, germanium, tin, lead, antimony and bismuth, all of these elements being able to alloy with lithium,
The above materials absorb lithium into crystal lattice thereof by charging, and desorb lithium by discharging. Repeated charge and discharge neither swells nor shrinks the skeletal crystal extensively, and fineness of the crystal particles progresses little. An excellent cycle characteristic thus can be expected.
And yet, the lithium quantity absorbed and desorbed into/from the crystal lattice is limited, those materials thus can not meet the request of the higher capacity, which still remains a problem.
A non-aqueous electrolyte secondary battery as a power supply of an apparatus is required the following features: 1. High capacity, 2. Long cycle life, 3. Chargeable/dischargeable with a high rate. In other words, if a negative electrode material has an extraordinary high capacity but has poor charge and discharge characteristics, it is impossible to reduce both size and weight of the power supply for some applications.
In the non-aqueous electrolyte secondary battery, an electrochemical activity of the particle surface of the negative electrode material is a key factor for improving the charge and discharge characteristics. For instance, in the negative electrode material comprising silicon as a host material, silicon has an advantage such as absorbing and desorbing a lot of lithium during charge-transfer reaction under high-rate charge and discharge. On the other hand, when one of suicides described previously is employed as the negative electrode material, an excellent cycle characteristic can be expected because of superior oxidation and reduction reaction during the charge and discharge according to the function of a mixed conductor for electrons and lithium ions. However, this material has an inferior characteristic of high-rate charge and discharge due to relatively smaller power of absorbing and desorbing lithium.